Serum electrolytes play a critical role in regulating normal physiologic functioning within and between cells. The testing of serum electrolytes is one of the most common analytical tests performed within hospitals. Such measurements are employed for routine monitoring of a patient as well as in emergency and life-threatening situations. Because of the vital role of electrolytes in normal physiologic responses, it is important that the measurement of the serum levels of electrolytes can be performed efficiently and accurately.
Sodium is one serum electrolyte critical in the physiologic control of water movement between the intracellular fluid compartment and the extracellular fluid compartment, i.e., maintaining osmotic pressure. In the healthy individual, the serum level of sodium is 135-145 mEq/l. Small deviations from normal level can have severe health consequences. An increased serum sodium can result from dehydration due to diarrhea or vomiting or nephrogenic diabetes. Low sodium levels usually are a result of too much water in the body, a condition associated with congestive heart failure, cirrhosis, nephritic syndrome, chronic renal failure, and syndrome of inappropriate anti-diuretic hormone (IADH).
Another source of electrolytes affecting physiologic function can also be those ions exogenously administered for therapeutic purposes. One example of such an ion is lithium. Therapeutic administration of lithium, typically as lithium carbonate, is one of the most effective agents for the treatment of patients suffering from bipolar disorder (manic depressive psychosis). Lithium acts by altering intraneuronal metabolism of catecholamines, inhibition of noradrenaline sensitive adenylate cyclase, and reduction in synaptic transmission and increase in neuronal excitability without modification of central nervous system (CNS) amine levels. Recently, studies have shown that lithium also holds promise in the treatment of Alzheimer's disease. However, lithium has severe toxic side effects. Toxicity is closely related to serum lithium levels and can occur at doses close to therapeutic levels, making the timely and accurate monitoring of serum levels critical. For example, serum Li+ levels over 1.5 mM (12 hours after a dose) usually indicate a significant risk of lithium toxicity.
Currently, the two most commonly used methods to detect serum sodium and lithium are ion-selective electrode (ISE) and flame photometry. ISE relies on ion-specific electrodes. Ideally, each electrode possesses a unique ion-selective property that allows it to respond to the desired ion. However, in practice, interference from other ions in the sample compromise the specificity of the detecting electrode, rendering the electrodes susceptible to false readings. The instrumentation for ISE is relatively expensive, requires routine maintenance that is sometimes cumbersome and time-consuming, and demands that the operating technician to have a considerable degree of skill and knowledge for accurate and consistent readings. Flame photometry relies on the principle that certain atoms, when energized by heat, become excited and emit a light of characteristic wavelength of radiant energy when returning to ground state. The intensity of the characteristic wavelength of radiant energy produced by atoms in the flame is directly proportional to the number of atoms excited in the flames, which is directly proportional to the concentration of the substance of interest in the sample. Like ISE, the instrumentation required for this method is complex and relatively expensive. Moreover, flame photometry requires the use of combustible gas, introducing sometimes expensive hazard prevention measures.
Conventional methods to detect sodium and lithium ions in samples are limited by complex instrumentation, potentially expensive and cumbersome maintenance, additional hazards, and often time requirements not suitable to emergency situations. The present invention addresses these problems and is more user friendly in automated analyzers.